A quick Binging (as opposed to Googling) uncovered an article about a golf ball warming device, in which a USGA official said that it wouldn’t be legal for tournament play to heat them during a round. You could, however, heat them prior.

So … boil the balls before heading out, then stick them in the insulated pocket of your bag.

I wonder how long balls retain heat. I’d need a thermal sensor for that.

Would a mesh sack be better or some sort of black vinyl sack which would attract as much solar heat as possible.

Also, is it illegal if I am wearing flannel lined pants, keep a handwarmer in my pocket for my hands and a ball in the same pocket? I will often rotate between two balls hole to hole just on chance anyway. Also, in my other pocket is my iPhone, I am not keeping a ball or handwarmer in that pocket.